| Nodular cast iron is an important metal structure material with excellent mechanical properties. To some extent, it can be a candidate for steel due to the similar properties but lower cost. The nodular cast iron are widely used in many fields, such as rail traffic, manufacturing, shipbuilding industry, and wind power equipment. Due to the poor working conditions in the above fields, the material is always suffering with frequently mechanical impact in cold or severe corrosion surroundings. Thus, higher toughness and strength of nodular cast iron are required. High strength and high toughness ductile cast iron, such as QT400-18, is the optimal material for the marine diesel engine cylinder block and cylinder head key castings, because of its superiority in the balance of strength and toughness. For the application in the practical manufacture, it is common method to use Si or solid solution strengthening alloy element along with high temperature graphitizing annealing to achieve both strength and toughness ductile iron castings.This topic is from the CRCC DALIAN CO., LTD where the author works, cooperating with dalian university of technology for casting branch of the practice subject. QT400-18 is the material for a large diesel engine cylinder block casting branch production in CRCC DALIAN CO., LTD, the technical requirements of which are Rm 390 MPa or higher tensile strength, yield strength 250 MPa Rp0.2 acuity, elongation A 15% or greater, brinell hardness HBS140-180, the average room temperature impact energy 14 J, single value or greater 11 J. In accordance with the generally high silicon+high temperature annealing process scheme, the production time will great increase for cleaning the oxide skin induced by high temperature annealing, with a result in wasting of manpower and resources. In order to shorten the production cycle and lower the cost, the chemical composition of molten iron, especially for the Si and Cu, are precisely controlled to improve the micro structure and mechanical properties, fitting all the requirements of a location block under as-cast casting standard.This topic is implemented in the process of production of P6 diesel engine block, high pure iron (from Benxi) and highquality carbon steel are chosen as the main material. They are melted in two 5T medium frequency induction furnace, by a new multi inoculation technique using the combination of pour-over laminated cover spheroidizing treatment and several inoculations. The result shows that the as-cast micro structure of this QT400-18 consists of ferrite,, nodular graphite, a very small amount of pearlite and hardening phase. On this basis, test of mechanical properties at three different positions was carried out. The result shows that the samples from the location in the casting end platform (lower surface mold) as marked 1 under the same cooling condition with the casting, has a dense microstructure without porosity or slag defects, as well as good performance and stability. Then the properties and microstructure of samples from the same place are studied with different silicon content (Sil.8%,2%,2.2%,2.6%) and different copper content (Cu0.05%,0.10%,0.16%,0.25%). The result shows that when the silicon content is 1.8%,the ferrite content was the lowest, as well as the strength and hardness, but the impact value at 23? was the highest; when the copper content is 0.25%, the content of pearlite was the highest as well as the strength and hardness, but the impact value at 23? was the lowest. According to the results, the composition of 1.8%Si+0.2% Cu is applied to P6 diesel engine production. Casting quality, and the economic benefit is remarkable.6 samples from the position 1 get the pearlite content state of 2%-10%, the tensile strength of 390MPa-410 MPa, the yield strength of 253MPa-266 MPa, the elongation of 20%-28%, Brinell hardness of HBS140-150, and the average impact of 14J-16J at room temperature, those can meet the technical requirements. |
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